This invention relates to methods for cleaning precision surfaces of wafers with supercritical fluids; and in particular to methods of cleaning wafers using rapid decompression techniques at supercritical state combined with full flow washing action.
Semiconductor device manufacturing or IC fabrication requires precisely controlled quantities of impurities to be introduced into tiny regions of the silicon substrate. Subsequently these regions must be interconnected to create components and electronic circuits. Lithographic processes create the patterns that define such regions. That is, a layer of photoresist materials is first spin-coated onto the wafer substrate. Next, this resist is selectively exposed to radiation such as ultraviolet light, electrons, or x-rays. An exposure machine, called stepper, and mask, called reticule, are used to effect the desired selective exposure. The patterns in the resist are formed when the wafer undergoes a subsequent development step. The areas of resist remaining after development protect the substrate regions, which they cover.
Locations from which resist has been removed can be subjected to a variety of subtractive (e.g. etching) or additive (e.g. ion implantation) processes that transfer the pattern onto the substrate surface. An advance integrated circuit can have up to 30 or more masking layers. Approximately one-third of the total cost of semiconductor manufacturing can be attributed to microlithographic processing; ref. Silicon Processing for the VLSI Era, S. Wolf and R. N. Tauber, Vol. 1, 2nd edition, Lattice Press, Sunset Beach, Calif., 2000, pp. 488, incorporated herein by reference.
The above mentioned residues are required to built up in the manufacturing process. Sidewall polymers, also called veils or via veils are a beneficial by-product, or artifact of RIE providing means for anisotropic etch to produce high aspect ratio vias. Compositionally sidewall polymers are generally low molecular weight fluoropolymers, that form from a combination of ion bombardment of the photoresist mask and fluorine in the etch gas chemistry. Upon back-sputtering from the via base metal ions are incorporated in the via veil, which tend to oxidize at high temperature (250xc2x0 C. to 400xc2x0 C.) oxygen plasma based photoresist stripping to become insoluble.
After etching (wet or dry) or ion implantation photoresist has to be removed. There are different degrees of difficulty required to do this, depending on the prior process. High-temperature hard bakes, plasma etch residues, sidewall polymers in contact holes and electrical interconnect trenches, ion implantation crusting, shrinking feature sizes, and new polymeric types of insulating material including low-k materials, all present challenges for the resist removal process. Both wet and dry stripping methods are being used. Plasma ashing as a dry method is currently the method of choice for the back-end of the fabrication process where the electrical interconnects are manufactured.
During plasma ashing, photoresist is removed by oxygen energized in a plasma field, which oxidizes the resist components to gases that are removed from the process chamber by a vacuum pump. Microwave, RF and UV-ozone sources generate the plasma. The disadvantage of plasma resist stripping is its ineffectiveness in the removal of metal ions and residues after dry etching, or reactive ion etching (RIE), namely sidewall polymers in vias, contact holes and trenches; ref. Microchip Fabrication, P. van Zant, 3rd edition, McGraw Hill, New York, 1997, pp. 273, incorporated herein by reference.
To complete the photoresist stripping process all residues have to be removed. This is typically done in wet chemical cleaning stations. After that the wafer has to be rinsed in deionized water and is finally dried. The process is called post-strip cleans.
The problems associated with the prior art methods of photoresist stripping by plasma ashing (PA) and residue removal can be summarized as follows:
PA is performed at high temperatures of 250xc2x0-400xc2x0 C., adding to the thermal budget of the wafer;
after PA residues are left in vias and trenches that need additional wet chemical treatment;
PA is not efficient for removing mobile metallic ion contamination;
PA can cause radiation damage of the electronic circuits;
PA of photoresist after ion implantation can lead to xe2x80x9cresist poppingxe2x80x9d littering the wafer with particulate matter;
with PA, selectivity between photoresist and low-k materials is bad and low-k material may be mechanically affected;
PA can modify the dielectric constant of low-k material due to charge damage;
post-strip wet chemical treatment may modify the low-k material in its electrical properties;
shrinking dimensions of features below 0.18 xcexcm present a problem for wet chemistry as post-strip cleans method, because of surface tension issues.
There are ongoing efforts to improve the efficiency of PA. A new process has been reported recently where during ashing fluorine-based gases are added that render residues water soluble. In a subsequent rinse step with deionized water the residues are being removed; ref. Manufacturing Qualification of an All Dry DeVeil Process, Richard Bersin, et al., The 197th Meeting of the Electrochemical Society, Toronto, 2000, incorporated herein by reference. Although the process is touted xe2x80x9call dryxe2x80x9d, a wet rinsing step has to follow and therefore problems with surface tension of water occur for small features as is typical for wet methods.
Alternative methods are being investigated. Some of them are still wet chemical based (Chilled Ozonexe2x88x92IMEC, Belgium; SO3+DI water rinsexe2x88x92Anon, USA) and therefore will have problems with processing small features. Laser stripping is a method developed in two modifications by different companies (Oramir, Israel; Radiance, USA). Both use laser energy to remove the photoresist. In one case environmentally critical process gases NF3 or CF4 are being added (Oramir), or inert gas is being used (Radiance). There is no indication as of today that these methods will find broad acceptance in the semiconductor industry. Other methods aim at the post-strip cleaning process with equipment either in the development phase or being used occasionally as post treatment tools (Dry Ice Spray xe2x80x94ATS Eco-Snow, USA; Charged Microclustersxe2x80x94Phrasor Scientific Inc., USA; Argon/Nitrogen Cryogenic Aerosol Cleaningxe2x80x94FSI International, USA).
Another technology being investigated for its obvious potential advantages is the use of supercritical fluids for wafer cleaning, particularly with carbon dioxide. This approach takes advantage of the supercritical state of a process chemical, which is another state of matter, also called phase, in addition to solid, liquid and gas. Sometimes the supercritical phase is referred to as xe2x80x9cdense gasxe2x80x9d, xe2x80x9ccompressible liquidxe2x80x9d, or xe2x80x9csupercritical fluidxe2x80x9d. There is considerable other art on the subject. However, there has been a dearth of understanding as to the best mode in which to apply the technology in a practical, easily automated process so as to produce the desired quality of cleaning, in the shortest amount of time, on a repetitive basis and at an affordable cost. It is this area of the art to which the following invention is directed.
In the description of the invention that follows, the supercritical phase of the process chemical is referred to as supercritical fluid. In this dense state the process substance fills the process chamber completely like a gas with the molecules interacting strongly. This leads to new properties of the process material that are crucial to the cleaning process. The supercritical state is characterized by a critical point, which constitutes of a critical pressure pc and a critical temperature Tc. At the critical point the density of the vapor and the liquid are identical. A material is in its supercritical state, if both, pressure and temperature are at or exceed the critical values.
Supercritical fluids have long been known for their abilities to dissolve organic contaminants. Their ability to display a wide range of solvent characteristics and the ability to tune solubility with small changes in temperature and pressure were identified early on. The gas-like diffusivity and low surface tension combined with liquid-like densities are important since these qualities enhance the cleaning effectiveness on parts, which have very small features (e.g. vias and trenches on semiconductor devices), or contain materials where selectivity of the supercritical fluid to one or the other component is a requirement, for example, between low-k material and photoresist in the fabrication of semiconductor devices; ref. Precision Cleaning With Supercritical fluid: A Case Study, John. E. Giles, et. al. in xe2x80x9cSupercritical Fluid Cleaningxe2x80x94Fundamentals, Technology and Applicationsxe2x80x9d, John McHardy, Samuel P. Sawan, ed., Noyes Publications, Westwood, N.J., USA, 1998, pp. 198, incorporated herein by reference
The effectiveness of supercritical fluids for cleaning can be improved. The use of binary and multi-component fluids is driven by a desire to either manipulate the critical temperature of the mixture, or to introduce polar or nonpolar features to regulate interactions of the fluid with a specific compound; ref. Supercritical Fluid Engineering Science, Erdogan Kiran, Joan F. Brennecke, ed., ACS Symposium Series 514, American Chemical Society, Washington, D.C. 1993, pp. 2, incorporated herein by reference.
Not withstanding all of the above, the problem of how to construct and operate an automated supercritical fluid cleaning process for satisfactory effect on semiconductor wafers and other precision surfaces remains unresolved in the prior art.
The invention, simply stated, is a process for cleaning precision surfaces such as the cleaning of photoresist off semiconductor wafers as part of the semiconductor fabrication process. The process relies on the use of process materials such as carbon dioxide that tend to be good solvents especially at supercritical temperature and pressure, alone or in combination with useful additives such as cosolvents and surfactants selected to shift the critical point downward or improve the cleaning effect. The process materials are applied to the substrate preferably exclusively in gaseous and supercritical states so as to avoid the problems associated with liquid contact.
Soak and agitation steps are applied to the substrate to aid both chemically and mechanically in the removal of the unwanted material from the substrate. The soak step permits infusion of the process materials into the unwanted matter at an elevated supercritical pressure. The agitation step includes a rapid decompression of the process chamber after the soak period, still within supercritical pressure, in order to mechanically weaken and break loose pieces of the photoresist, sidewall polymer and such other materials as are sought to be removed, with a very significant pressure differential. This is combined with a supercritical fluid flush to carry away the loose debris, and is then preferably concluded by rapidly elevating the vessel pressure back to the higher supercritical pressure, stressing the unwanted material this time with rapid compression. The core process steps are preceded and followed by more conventional loading and unloading steps, except that the purging and pressurization steps avoid any liquid contact with the substrate, constraining the inflowing process materials to process gas and supercritical fluid.
It is therefore an objective of the invention to provide a dry process for cleaning of precision surfaces using supercritical fluids such as carbon dioxide, alone or in combination with suitable additives such as cosolvents and surfactants. It is a further object to agitate a substrate after suitable exposure to a supercritical fluid or mixture under a higher supercritical pressure, with a rapid decompression of a significant degree. It is a yet further object to combine a rapid decompression step with a full flow supercritical fluid or mixture flush or wash of the substrate, in order to further loosen and evaluate any loose debris, and to further expose the unwanted materials on the substrate to the solvent effects of the process materials. It is another object to illustrate the salient points of an apparatus by which the process may be conducted.
What follows is a preferred embodiment process only, and should not be interpreted as limiting of the invention.